1. Field of the Invention
The invention generally relates to laser light generating apparatuses. More particularly, the invention relates to a laser light generating apparatus including a laser light source and a plurality of external resonators, for achieving stable locking of the apparatus utilizing a single modulation signal.
2. Description of the Related Art
An FM sideband technology (“Pound-Drever-Hall Locking” technology) is typically known as an example of a technology for stable locking of an external resonator in a laser light generating apparatus, and is widely used for locking external resonators.
In the FM sideband method, sideband waves are generated using a phase-modulator disposed at a stage preceding an external resonator to generate an error signal. In a case where the sideband waves are generated by modulating ultraviolet light, in particular, a high-performance phase-modulator is desirable, having a high light transmittance and low operating voltages.
According to the related art technology, to avoid undue damages of the phase-modulator, which are caused by the ultraviolet light, the locking of the ultraviolet light is carried out by structuring a laser light generating apparatus including a plurality of external resonators, locking simultaneously the plurality of external resonators according to the FM sideband technology, and implementing wavelength conversion over multiple stages. When two external resonators are used, for example, the abovementioned steps are carried out by first disposing a first phase-modulator at the stage preceding the first external resonator to be subjected to locking according to the FM sideband technology, allowing the light subjected to wavelength conversion by the first external resonator (i.e., generation of a wavelength-converted light), to be incident on a second phase-modulator, allowing light emerging from the second phase-modulator to be incident on a second external resonator, and subsequently implementing the locking of the second external resonator, followed by another wavelength conversion.
However, in the abovementioned arrangement having a plurality of resonators to be locked simultaneously, there are utilized as many phase-modulators as the external resonators, since one phase-modulator is disposed at the stage preceding each of the external resonators. In addition, the phase-modulator is relatively expensive to make because of optical crystals (nonlinear optical elements) included therein as major components. Therefore, an increase in the number of resonators has resulted in unduly increase in size as a whole and cost of the apparatus.
Further, in the case of ultraviolet light as the light output following the wavelength conversion, a relatively small number of phase-modulators are operable with the ultraviolet light, having low light transmittance and inferior figure of merit. This results in several difficulties such as the provision of a high voltage power source for supplying high operating voltages and a relatively short life span due to damages caused by the ultraviolet light.
Japanese Unexamined Patent Application Publication No. 2002-311467 discloses a device and a method, in which multiple-stage locking is carried out by inputting plural modulation signals (carriers) to a phase-modulator.
Although the device and method described in the abovementioned application publication are capable of compensating some of previous difficulties, several points are yet to be addressed such as the use of a plurality of modulation signal generation units for supplying a plurality of modulation signals and increased complication of signal processing in the device and method.
As to the method of locking external resonators at multiple stages using one single modulation signal, it is desirable to stabilize the locking (resonance state) by increasing the proportion of modulated light reflected from an external resonator at the first stage as much as possible so as to increase S/N ratios of error signals, and simultaneously by allowing the modulated light to pass through the external resonator at the first stage as much as possible so as to increase S/N ratios of error signals detected by other external resonators at each of the succeeding stages. Thus, this gives rise to tradeoff in view of fulfilling simultaneously both abovementioned conditions for respective external resonators.
Therefore, when external resonators as a whole are not optimized in early stages following setting renewal, or the change over time in transmission widths of external resonators takes place due to changes in resonator loss and in conversion efficiency, several difficulties of running into instability of the locking have been encountered for a system including plural external resonators.
In addition, in order to increase wavelength conversion efficiency or to obtain a high-power wavelength converted light, a high-power laser light has to irradiate external resonators to implement the wavelength conversion of the laser light. Although this irradiation is carried out by letting the high power laser light to be incident on a phase-modulator, the intensity of the incident laser light can be increased only to a certain limit of damage for the phase-modulator because of possible damaging effects by the laser light irradiation. The damages may be avoided by using the phase-modulator provided with components having a large effective diameter. However, this takes an optical crystal (nonlinear optical element) in the phase-modulator to be increased in size and accordingly in its costs. Therefore, it is highly desirable to be able to carry out stable signal modulations utilizing low intensity laser light.